This invention relates to load cells and in particular to an improved double-ended, centre-loaded beam-type load cell for measuring force and the method in which such a load cell is mounted.
It is common to construct load cells in a double-ended beam configuration, and to utilize electrical strain gages which are installed upon surfaces of the beam in various configurations to measure the strain occurring in such a beam under an applied force. The purpose of the strain gages is to respond to changes in the force-induced strain and to provide, when connected in the commonly used Wheatstone bridge circuit, a change in electrical output signal representative of a change in force applied to the load cell. Such arrangements are well known in the art.
U.S. Pat. No. 3,879,998 (Bradley) discloses a double-ended beam load cell which utilizes strain gages which are mounted and are electrically connected in a differential-moment configuration. This load cell is constructed of a beam of constant diameter, and therefore requires the use of fabricated force-receiving mounting parts which are externally clamped onto the load cell. Such force-receiving mounting sections are relatively bulky, tall, and heavy, as well as somewhat expensive to construct. Also, due to their being locked to the beam, unwanted strains may be induced within the load cell through imperfect mounting tolerances causing torsion and/or longitudinal couples to be applied to the load cell, which reduces measurement accuracy. Although the strain gages are generally configured in a way so as to cancel such unwanted extraneous forces, such cancellation is rarely perfect due to small misalignments of the strain gages.
Double-ended beam load cells are also constructed with the well known shear beam strain sensing configuration, such as the xe2x80x9cRL75016 double-ended shear beam load cellxe2x80x9d from Rice Lake Weighing Systems. The mounting method is similar to that disclosed above, and the same undesirable strains are induced, due to similar undesirable load-receiving mounting sections. As well, the force-receiving mounting parts are also bulky, tall, and heavy, and expensive to construct.
The same problems described above regarding differential-moment and shear-beam double-ended beams also apply to parallelogram-beam double-ended beams. The fundamentals of differential-moment, shear-beam, and parallelogram-beam force-sensing principles are well known in prior art.
It is an aspect of an object of the present invention to reduce the vertical height, weight, bulk, and machining cost of a load cell and related mounting components.
It is an aspect of a further object of the present invention to provide a strain gage load cell with a simplified mounting method.
It is an aspect of a further object of the present invention to provide a load cell with a lower-cost mounting method.
It is an aspect of a further object of the present invention to provide a load cell and mount which is lower in height.
It is an aspect of a further object of the present invention to provide a load cell and mount which together and separately are lower in bulk and weight.
It is an aspect of the present invention to provide a load cell that can more easily be mounted using commonly available link components (such as those manufactured by The Crosby Group of USA, xe2x80x9cGrade 8 Alloy Fittingsxe2x80x9d, Page 131, The Crosby Group.).
It is an aspect of the present invention to provide a load cell for use in a reciprocating-motion oil well, in which the sum of the weight and the vertical forces acting upon a down-hole rod string, are supported by and therefore may be measured by the load cell.
It is an aspect of the present invention to provide a load cell with alternative mounting methods which utilize bearings or bushings for a more repeatable and durable wear of usage in the rotational directions accorded to each bearing""s employment, which is of benefit in typical installations such as truck-borne tipping gear (dump bodies), so as to provide a fully supporting, self-hinging, load weighing assembly.
It is an aspect of the present invention to provide a load cell with a centre load-receiving section with a centre diameter which is substantially greater than the diameter of the force-reacting sections, and which centre diameter is large enough to allow a hole for a pin of sufficient load-carrying capacity with respect to the capacity of the load cell to be machined through the centre force receiving section.
It is an aspect of the present invention to provide a load cell with pin-loading, which substantially eliminates the introduction of longitudinal couples due to allowed rotation about the rotational axis of the pin.
It is an aspect of the present invention to provide a load cell having transition areas, along the length of the beam, from the larger diameter middle section, to the smaller-diameter strain-reacting sections, that are gradual in nature in order to reduce stress concentrations in the transition areas.
It is an aspect of the present invention to provide a load cell that can be constructed using differential-moment, parallelogram-beam, or shear-strain sensing methods.
It is an aspect of the present invention to provide a load cell having a longitudinal strength of the load cell body that is large in comparison with the vertical load forces intended to be measured, due to the nature of most materials, such as steel, to have their greatest strength when loaded in compression or tension. This is consistent with parallelogram-beam, shear-beam, and differential-moment-beam load cells.
It is an aspect of the present invention to provide clearance distances in a longitudinal direction between a load cell and each end mounting member that are sufficient as to overcome the longitudinal displacement due to thermal or other incidental and normal expansion of adjoining structures such as foundations, mounted containers, or weigh scale decks.
It is an aspect of the present invention to provide a method of mounting adjacent load cells where the mounts are mutually angularly orientated away from one another according to the number of load cells employed in an installation. The mutual angular displacements are such that the long axis of the load cell and mount point toward the centre of the supported structure, or may be aligned such that the long axis of the load cell and mounts are tangential to the centre of the supported structure.
It is an aspect of the present invention to provide a method of mounting load cells such that clearance distances between each of the surfaces of any mounting member contacting, resting upon, and loading the pin, are substantially greater than the longitudinal clearances of the preceding paragraph, such that sideways forces applied to the load cells are substantially reduced.
It is an aspect of the present invention to provide mounts which are employed one at each end of the load cell, and which provide supporting surfaces which are concentric with respect to the longitudinal axis of the load cell, which allows the load cell to rotate freely within each mount, thus eliminating the introduction of torsional forces to the load cell. The forces acting upon each end of the pin thus cause the load cell to rotate until the forces are equal at both ends of the pin. In this state of equilibrium, the load cell does not rotate freely with regard to the applied forces. In this way, the load cell""s force sensing axis is always aligned with the direction of the applied forces.
It is an aspect of the present invention to provide a method of constructing load cells where a major initial material removal to create the shape of the load cell is by lathe turning, which is fundamentally less costly and more efficient than other machining operations, such as milling. A disadvantage of this is that more overall material may be removed than with other designs, however, with large amounts of recycling of waste material being commonplace in the machining industry, any such loss of material is significantly mitigated.
It is an aspect of the present invention to provide mounts that are preferably made of metal such as steel or aluminum, and may be constructed using either casting or welded fabrication methods, or a combination of both.
These together with other aspects and advantages which will be subsequently apparent, reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.